Why is there no dieresis (**)over the letter "e" in the word dieresis?

Answer 1

First, if you use the older spellings -- diaresis and esp.diæresis (with or without the ligature of a+e) -- the mark is not at all necessary, since there is no chance of the vowels being misread as a diphthong.

As for "dieresis", the preferred American English spelling, well the trend has been away from using the mark at all! (It is rather rare to see "coöperate" or "naïve", for instance.) So why would we suddenly add a new one where there had not been one before? And if you KNOW the word, you're not very likely to confuse it with anything else.

In my experience, the mark only seems to appear when two forms need to be distinguished (e.g. the greeting Noël vs. the male name Noel) and perhaps in a name like Chloë or Brontë, but there's a stronger tradition of keeping old spellings of proper names than of words in general.

Answer 2

Because the pronunciation of the word does not call for one. There is no tilde ~ in the word tilde, either. That would be cool, though.